Waste heat recovery system

ABSTRACT

The waste heat recovery system of an engine includes a Rankine cycle, a first bypass passage, a first valve and a control unit. The Rankine cycle allows a working fluid to circulate therethrough. The Rankine cycle has a first heat exchanger, a second heat exchanger, an expander and a condenser. The first heat exchanger exchange heat between the working fluid and the engine or a first intermediate medium exchanging heat with the engine. The first bypass passage allows the working fluid to pass therethrough. One end of the first bypass passage is located at an upstream side of the condenser and the other end is located at a downstream side of the condenser. The first valve opens and closes the first bypass passage. When temperature of the engine or the first intermediate medium is lower than a first predetermined value, the control unit opens the first valve.

BACKGROUND OF THE INVENTION

The present invention relates to a waste heat recovery system and more particularly to a waste heat recovery system using a Rankine cycle.

The waste heat recovery system using a Rankine cycle has been developed to recover mechanical energy (power) from waste heat of a vehicle engine. The Rankine cycle generally includes a pump that pumps working fluid, a heat exchanger that heats working fluid by the waste heat of an engine, an expander that expands the heated working fluid to recover mechanical energy, and a condenser that condenses the expanded working fluid.

Japanese Patent Application Publication No. 2007-85195 discloses a waste heat recovery system having two heat exchangers. The Rankine cycle of this waste heat recovery system includes a first heat exchanger that heats working fluid by heat exchange with a cooling water of the engine, and a second heat exchanger that heats the working fluid by heat exchange with exhaust gas of the engine. The first heat exchanger serves as a cooling water boiler and the second heat exchanger serves as an exhaust gas boiler. The working fluid delivered by the pump absorbs heat while flowing through the first heat exchanger and the second heat exchanger, generates mechanical energy while flowing through the expander, and releases heat while flowing through the condenser.

When the temperature of the cooling water of the engine is low as in the case of a start-up of the engine, the fuel efficiency of the engine generally deteriorates. In the waste heat recovery system of the cited reference, if the heat absorbed by the working fluid in flowing through the first heat exchanger is transferred to the cooling water to raise the temperature of the cooling water rapidly, the fuel efficiency of the engine is improved. However, the working fluid flowing through the Rankine cycle releases the heat from the condenser. Therefore, the temperature of the cooling water is not raised rapidly and the fuel efficiency of the engine deteriorates.

The present invention is directed to a waste heat recovery system, wherein when the temperature of the cooling water of the engine is low, the fuel efficiency of the engine is improved by raising the temperature of the cooling water rapidly.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, the waste heat recovery system of an engine includes a Rankine cycle, a first bypass passage, a first valve and a control unit. The Rankine cycle allows a working fluid to circulate therethrough. The Rankine cycle has a first heat exchanger, a second heat exchanger, an expander and a condenser. The first heat exchanger is provided for exchanging heat between the working fluid and the engine or a first intermediate medium exchanging heat with the engine. The second heat exchanger is provided for exchanging heat between the working fluid and an exhaust gas of the engine or a second intermediate medium exchanging heat with the exhaust gas of the engine. The expander is provided for expanding the working fluid to recover mechanical energy. The condenser is provided for condensing the working fluid. The first bypass passage is connected to the Rankine cycle for allowing the working fluid to pass therethrough. One end of the first bypass passage is located at an upstream side of the condenser and the other end of the first bypass passage is located at a downstream side of the condenser with respect to flow direction of the working fluid. The first valve is provided for opening and closing the first bypass passage. The control unit is provided for controlling operation of the waste heat recovery system. When temperature of the engine or the first intermediate medium is lower than a first predetermined value, the control unit opens the first valve.

Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

FIG. 1 is a schematic view showing a waste heat recovery system according to a first embodiment of the present invention;

FIG. 2 is a schematic view showing a waste heat recovery system according to a second embodiment of the present invention;

FIG. 3 is a schematic view showing a waste heat recovery system according to a third embodiment of the present invention;

FIG. 4 is a schematic view showing a waste heat recovery system according to a fourth embodiment of the present invention;

FIG. 5 is a schematic view showing a waste heat recovery system according to a modification of the first embodiment of the present invention;

FIG. 6 is a schematic view showing a waste heat recovery system according to another modification of the first embodiment of the present invention;

FIG. 7 is a schematic view showing a waste heat recovery system according to yet another modification of the first embodiment of the present invention; and

FIG. 8 is a schematic view showing a waste heat recovery system according to a modification of the third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe the embodiments of the present invention with reference to the accompanying drawings. FIG. 1 shows a waste heat recovery system 100 according to the first embodiment of the present invention. The waste heat recovery system 100 includes a Rankine cycle 110. The Rankine cycle 110 has a gear pump 111, a cooling water boiler 112, an exhaust gas boiler 113, an expander 114 and a condenser 115. The gear pump 111 is provided for pumping a working fluid. The cooling water boiler 112 is provided for exchanging heat between the working fluid and a cooling water exchanging heat with an engine 140, serving as the first heat exchanger of the present invention. The exhaust gas boiler 113 is provided for exchanging heat between the working fluid and an exhaust gas of the engine 140, serving as the second heat exchanger of the present invention. The expander 114 is provided for expanding the working fluid heated and vaporized by the cooling water boiler 112 and the exhaust gas boiler 113 thereby to generate mechanical energy (power). The condenser 115 is provided for condensing the expanded working fluid. The gear pump 111, the cooling water boiler 112, the exhaust gas boiler 113, the expander 114 and the condenser 115 are connected in this order to form a closed circuit. The cooling water for cooling the engine 140 serves as the first intermediate medium for heating the cooling water boiler 112. Exhaust gas from the engine 140 serves as the medium for heating the exhaust gas boiler 113. Exhaust gas which is high in temperature heats the working fluid rapidly after the engine 140 is started. Therefore, the exhaust gas boiler 113 can heat the working fluid more rapidly than the cooling water boiler 112. The cooling water of the engine 140 flows in a cooling water circuit a in which the engine 140, the cooling water boiler 112 and a radiator 130 are provided.

The expander 114 has an output shaft 114A which is driven to rotate by mechanical energy generated in expanding the vaporized working fluid in the expander 114. A motor generator 116 is connected to the output shaft 114A for converting the rotary drive force to electric power. The gear pump 111 has a drive shaft 111A which is connected to the motor generator 116.

A first bypass passage 117 is connected to the Rankine cycle 110 for allowing the working fluid to pass through the first bypass passage 117 so as to bypass the condenser 115. One end of the first bypass passage 117 is located at the upstream side of the condenser 115 and the downstream side of the expander 114, as viewed in the flow direction of the working fluid. The other end of the first bypass passage 117 is located at the downstream side of the condenser 115 and the upstream side of the gear pump 111. Pressure loss of working fluid flowing through the first bypass passage 117 is sufficiently smaller than that of the working fluid flowing through the condenser 115. An electromagnetic valve 118 is provided at an intermediate position of the first bypass passage 117 for opening and closing the first bypass passage 117, serving as the first valve of the present invention. The electromagnetic valve 118 is electrically connected to a control unit 150 that controls the operation of the waste heat recovery system 100. The control unit 150 controls the operation of the electromagnetic valve 118 in accordance with temperature information obtained by a temperature sensor 160 that measures the temperature of the cooling water at the downstream side of the engine 140 as viewed in the flow direction of the cooling water. Thus, the first bypass passage 117 is opened or closed by the electromagnetic valve 118.

A second bypass passage 119 is connected to the Rankine cycle 110 for allowing the working fluid to pass through the second bypass passage 119 so as to bypass the condenser 115 and the gear pump 111. One end of the second bypass passage 119 is located at the downstream side of the expander 114 and the upstream side of the gear pump 111. The other end of the second bypass passage 119 is located at the downstream side of the gear pump 111 and the upstream side of the cooling water boiler 112. Although in the present embodiment the second bypass passage 119 bypasses the condenser 115 and the gear pump 111, the second bypass passage 119 may bypass only the gear pump 111. A check valve 120 is provided at an intermediate position of the second bypass passage 119 for opening and closing the second bypass passage 119, serving as the second valve of the present invention. The check valve 120 opens the second bypass passage 119 when the pressure P1 of working fluid at the upstream side of the gear pump 111 is higher than the pressure P2 of working fluid at the downstream side of the gear pump 111 (P1>P2).

The following will describe the operation of the waste heat recovery system 100 of the first embodiment. The description will be made for two cases, i.e. one case where the temperature of the cooling water of the engine 140 is a first predetermined value Th1 or higher and the other case where the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1.

When the temperature of the cooling water of the engine 140 obtained by the temperature sensor 160 is the first predetermined value Th1 or higher, the control unit 150 causes the electromagnetic valve 118 to close thereby to close the first bypass passage 117.

With the first bypass passage 117 closed, the working fluid delivered by the gear pump 111 is evaporated into a high-temperature gas by absorption of heat from the cooling water and exhaust gas of the engine 140 while flowing through the cooling water boiler 112 and the exhaust gas boiler 113. The evaporated working fluid is then expanded by the expander 114. The pressure P1 of working fluid at the upstream side of the gear pump 111 is lower than the pressure P2 of working fluid at the downstream side of the gear pump 111 (P1<P2), so that the check valve 120 is closed and, therefore, the second bypass passage 119 is closed. In addition, the first bypass passage 117 is also closed. Thus, the working fluid flowed out of the expander 114 flows into the condenser 115. The working fluid releases heat while being condensed by the condenser 115 and then is transferred to the cooling water boiler 112 by the gear pump 111. The expander 114 is driven by the expansion of the working fluid. The mechanical energy generated by the expansion also drives the motor generator 116 and the gear pump 111.

When the temperature of the cooling water of the engine 140 obtained by the temperature sensor 160 is lower than the first predetermined value Th1, on the other hand, the control unit 150 causes the electromagnetic valve 118 to open thereby to open the first bypass passage 117, thus allowing the working fluid to circulate through the first bypass passage 117. The expander 114 and the gear pump 111 are driven by the mechanical energy generated by the motor generator 116.

With the first bypass passage 117 thus opened, the working fluid delivered by the gear pump 111 is evaporated by absorption of heat from the exhaust gas of the engine 140 while flowing through the exhaust gas boiler 113 after flowing through the cooling water boiler 112. The evaporated working fluid is then expanded by the expander 114. With the first bypass passage 117 opened, the pressure loss of working fluid flowing through the first bypass passage 117 is sufficiently smaller than that of working fluid flowing through the condenser 115. Therefore, most of the working fluid passes through the first bypass passage 117 rather than bypassing the condenser 115 and flowed into the gear pump 111. Since the inlet capacity of the expander 114, or the volume of working fluid to be drawn into the expander 14, is larger than that of the gear pump 111, or the volume of working fluid to be drawn into the gear pump 11, the pressure P1 of working fluid at the upstream side of the gear pump 111 and the downstream side of the expander 114 exceeds the pressure P2 of working fluid at the downstream side of the gear pump 111 and the upstream side of the cooling water boiler 112 (P1>P2). As a result, the check valve 120 is opened thereby to open the second bypass passage 119. In such a state of the Rankine cycle 110, the working fluid flowed out of the expander 114 is divided into two ways, one flowing through the gear pump 111 via the first bypass passage 117 and the other flowing through the second bypass passage 119 so as to bypass the gear pump 111. High temperature working fluid flowed into the cooling water boiler 112 transfers its heat to the cooling water of the engine 140 while flowing through the cooling water boiler 112. Thus, the temperature of the cooling water of the engine 140 is raised rapidly thereby to improve the fuel efficiency of the engine 140.

According to the waste heat recovery system 100 of the first embodiment, when the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1, circulation of working fluid through the first bypass passage 117 is allowed. The expander 114, the first bypass passage 117, the gear pump 111, the cooling water boiler 112 and the exhaust gas boiler 113 cooperate to form a circulation circuit. Thus, the working fluid bypassing the condenser 115 is delivered by the expander 114 and then supplied to the cooling water boiler 112. Therefore, when the temperature of the cooling water of the engine 140 is still low as in the case of a start-up of the engine 140, the temperature of the cooling water of the engine 140 is raised rapidly thereby to improve the fuel efficiency of the engine 140.

When the pressure P1 of working fluid at the upstream side of the gear pump 111 exceeds the pressure P2 of working fluid at the downstream side of the gear pump 111 (P1>P2), the second bypass passage 119 is opened. Since part of the working fluid delivered by the expander 114 bypasses the gear pump 111 which may cause a pressure loss, the pressure loss of the working fluid due to the gear pump 111 is avoided.

FIG. 2 shows a waste heat recovery system 200 according to the second embodiment of the present invention. Although the first bypass passage 117 of the first embodiment bypasses only the condenser 115, the first bypass passage 217 of the second embodiment bypasses the expander 114, as well as the condenser 115. For the sake of convenience of explanation, like or same parts or elements in the subsequent embodiments will be referred to by the same reference numerals as those which have been used in the first embodiment, and the description thereof will be omitted.

The first bypass passage 217 is connected to the Rankine cycle 210 so as to bypass the expander 114 and the condenser 115. One end of the first bypass passage 217 is located at the upstream side of the expander 114 and the downstream side of the exhaust gas boiler 113. The other end of the first bypass passage 217 is located at the downstream side of the condenser 115 and the upstream side of the gear pump 111. The pressure loss of working fluid flowing through the first bypass passage 217 is sufficiently smaller than that of working fluid flowing through the expander 114 and also that of working fluid flowing through the condenser 115. An electromagnetic valve 218 is provided at an intermediate position of the first bypass passage 217 for opening and closing the first bypass passage 217, serving as the first valve of the present invention. The electromagnetic valve 218 is electrically connected to the control unit 250 that controls the operation of the waste heat recovery system 200.

A one-way clutch 221 is mounted on the output shaft 114A at a position between the expander 114 and the motor generator 116, serving as the clutch of the present invention. The one-way clutch 221 transmits the rotary drive force from the expander 114 to the gear pump 111, but blocks the transmission of the rotary drive force from the gear pump 111 to the expander 114.

The following will describe the operation of the waste heat recovery system 200 of the second embodiment. The description will be made for two cases, i.e. one case where the temperature of the cooling water of the engine 140 is the first predetermined value Th1 or higher and the other case where the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1.

When the temperature of the cooling water of the engine 140 obtained by the temperature sensor 160 is the first predetermined value Th1 or higher, the control unit 250 causes the electromagnetic valve 218 to close thereby to close the first bypass passage 217.

With the first bypass passage 217 closed, the working fluid delivered from the gear pump 111 circulates through the cooling water boiler 112, the exhaust gas boiler 113, the expander 114 and the condenser 115 in this order in the Rankine cycle 210. The output shaft 114A of the expander 114 is driven to rotate by the expansion of the working fluid. The one-way clutch 221 transmits rotary drive force from the expander 114 to the gear pump 111. The motor generator 116 and the gear pump 111 are driven by the mechanical energy generated by the expander 114.

When the temperature of the cooling water of the engine 140 obtained by the temperature sensor 160 is lower than the first predetermined value Th1, on the other hand, the control unit 250 causes the electromagnetic valve 218 to open thereby to open the first bypass passage 217, thus allowing the working fluid to circulate through the first bypass passage 217. The gear pump 111 is driven by the mechanical energy generated by the motor generator 116.

With the first bypass passage 217 thus opened, the working fluid delivered by the gear pump 111 is evaporated by absorption of heat from the exhaust gas of the engine 140 while flowing through the exhaust gas boiler 113 after flowing through the cooling water boiler 112. Since the first bypass passage 217 is opened and the expander 114 is at a stop (which will be described later), most of the working fluid flowed out of the exhaust gas boiler 113 is drawn into the gear pump 111 via the first bypass passage 217 without flowing through the expander 114. As a result, the entire pressure of the working fluid flowing through the Rankine cycle 210 is substantially equalized and the working fluid is flowed into the cooling water boiler 112 under little pressure difference. Since the output shaft 114A of the expander 114 then receives no rotary drive force, the gear pump 111 is driven by the mechanical energy generated by the motor generator 116. The one-way clutch 221 blocks the transmission of rotary drive force from the gear pump 111 to the expander 114. High temperature working fluid pumped into the cooling water boiler 112 transfers its heat to the cooling water of the engine 140 while flowing through the cooling water boiler 112. Thus, the temperature of the cooling water of the engine 140 is raised rapidly thereby to improve the fuel efficiency of the engine 140.

According to the waste heat recovery system 200 of the second embodiment, when the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1, the working fluid is allowed to circulate through the first bypass passage 217. The gear pump 111, the cooling water boiler 112, the exhaust gas boiler 113 and the first bypass passage 217 cooperate to form a circulation circuit. Thus, the working fluid bypassing the expander 114 and the condenser 115 is supplied to the cooling water boiler 112. Therefore, when the temperature of the cooling water of the engine 140 is low as in the case where the engine 140 is started, the temperature of the cooling water of the engine 140 is raised rapidly thereby to improve the fuel efficiency of the engine 140.

The entire pressure of the working fluid flowing through the Rankine cycle 210 is substantially equalized, so that the working fluid is flowed into the cooling water boiler 112 under little pressure difference.

The one-way clutch 221 blocks the transmission of the rotary drive force from the gear pump 111 to the expander 114, thus preventing the expander 114 from transferring the working fluid to the condenser 115.

FIG. 3 shows a waste heat recovery system 300 according to the third embodiment of the present invention. The waste heat recovery system 300 of the third embodiment includes a first bypass passage 317 that bypasses the condenser 115 and the gear pump 111, and a third bypass passage 322 that bypasses the cooling water boiler 112, the exhaust gas boiler 113 and the expander 114. The expander 114, the first bypass passage 317, the cooling water boiler 112 and the exhaust gas boiler 113 cooperate to form a first closed circuit A and the gear pump 111, the third bypass passage 322 and the condenser 115 cooperate to form a second closed circuit B. In addition, there is a hysteresis of the temperature of the cooling water that is used as the criterion for controlling the opening and closing of the first bypass passage 317 and the third bypass passage 322.

The first bypass passage 317 is connected to the Rankine cycle 310 so as to bypass the condenser 115 and the gear pump 111. One end of the first bypass passage 317 is located at the upstream side of the condenser 115 and the downstream side of the expander 114. The other end of the first bypass passage 317 is located at the downstream side of the gear pump 111 and the upstream side of the cooling water boiler 112. The pressure loss of working fluid flowing through the first bypass passage 317 is sufficiently smaller than that of working fluid flowing through the condenser 115 and also that of working fluid flowing through the gear pump 111. A first three-way valve 318 is provided at the aforementioned one end of the first bypass passage 317 for opening and closing the first bypass passage 317, serving as the first valve of the present invention. The first three-way valve 318 is electrically connected to the control unit 350 that controls the operation of the waste heat recovery system 300. The first three-way valve 318 functions to switch the direction of the working fluid flowed out of the expander 114 to either the condenser 115 or the first bypass passage 317.

The third bypass passage 322 is connected to the Rankine cycle 310 for allowing the working fluid to pass through the third bypass passage 322 so as to bypass the condenser 115 and the gear pump 111. One end of the third bypass passage 322 is located at the downstream side of the one end of the first bypass passage 317 and the upstream side of the condenser 115. The other end of the third bypass passage 322 is located at the upstream side of the other end of the first bypass passage 317 and the downstream side of the gear pump 111. A second three-way valve 323 is provided at the other end of the third bypass passage 322 for opening and closing the third bypass passage 322, serving as the third valve of the present invention. The second three-way valve 323 is electrically connected to the control unit 350. The second three-way valve 323 functions to switch the direction of the working fluid flowed out of the gear pump 111 to either the cooling water boiler 112 or the third bypass passage 322.

In addition, the cooling water passage coming out from the engine 140 is divided at the downstream side of the engine 140 into two cooling water circuits, one circuit α1 that includes the cooling water boiler 112 and the other circuit α2 that includes the radiator 130. The opening and closing of the cooling water circuits a1 and a2 are switched by a thermostat 324 that is operable in accordance with the temperature of the cooling water of the engine 140.

The following will describe the operation of the waste heat recovery system 300 of the third embodiment. The description will be made for two cases, i.e. one case where the temperature of the cooling water of the engine 140 is a second predetermined value Th2 that is slightly higher, e.g. by two degrees, than the aforementioned first predetermined value Th1 or higher and the other case where the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1.

When the temperature of the cooling water of the engine 140 is the second predetermined value Th2 or higher, the control unit 350 moves the first three-way valve 318 to such a position that allows the working fluid flowed out of the expander 114 into the condenser 115. That is, the first bypass passage 317 is closed thereby to cut off the circulation of the working fluid through the first bypass passage 317. In addition, the control unit 350 moves the second three-way valve 323 to such a position that allows the working fluid pumped out of the gear pump 111 into the cooling water boiler 112. That is, the third bypass passage 322 is closed thereby to cut off the circulation of the working fluid through the third bypass passage 322.

In this case, the working fluid delivered by the gear pump 111 circulates through the cooling water boiler 112, the exhaust gas boiler 113, the expander 114 and the condenser 115 in this order in the Rankine cycle 310. The expander 114 is driven by the expansion of the working fluid. The mechanical energy generated by the expansion drives the motor generator 116 and the gear pump 111.

When the temperature of the cooling water of the engine 140 is a third predetermined value Th3 that is higher than the second predetermined value Th2, or higher, the thermostat 324 is operated to close the cooling water circuit α1 and open the cooling water circuit α2. When the temperature of the cooling water of the engine 140 is lower than the third predetermined value Th3, the thermostat 324 is operated to open the cooling water circuit α1 and close the cooling water circuit α2. Thus, when the temperature of the cooling water of the engine 140 is the third predetermined value Th3 or higher, the thermostat 324 serves to close the cooling water circuit α1 that includes the cooling water boiler 112 and to open the cooling water circuit α2 that includes the radiator 130. Therefore, the waste heat of the engine 140 is released from the radiator 130. Any recoverable heat of high-temperature cooling water of the engine 140 should be recovered by the Rankine cycle 310 for improvement of fuel efficiency of the engine 140. Therefore, the cooling water circuit α1 is not completely closed, but it is slightly opened.

When the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1, the control unit 350 moves the first three-way valve 318 to a position where the working fluid flowed out of the expander 114 is allowed into the first bypass passage 317. That is, the expander 114, the first bypass passage 317, the cooling water boiler 112 and the exhaust gas boiler 113 then cooperate to form the first closed circuit A with the first bypass passage 317 opened. In addition, the control unit 350 moves the second three-way valve 323 to a position where the working fluid pumped out of the gear pump 111 is allowed into the third bypass passage 322. That is, the gear pump 111, the third bypass passage 322 and the condenser 115 then cooperate to form the second closed circuit B with the third bypass passage 322 opened.

In this case, the working fluid circulating through the first closed circuit A is evaporated by absorption of heat from the exhaust gas of the engine 140 while flowing through the exhaust gas boiler 113. The evaporated working fluid is then transferred to the expander 114 to be expanded. High-temperature working fluid flowed to the cooling water boiler 112 through the first bypass passage 317 transfers its heat to the cooling water of the engine 140 while flowing through the cooling water boiler 112. Thus, the temperature of the cooling water of the engine 140 is raised rapidly thereby to improve the fuel efficiency of the engine 140.

The working fluid circulating through the second closed circuit B is flowed through the third bypass passage 322 by the gear pump 111. The working fluid absorbs heat from the expander 114 at a position adjacent to the outlet of the third bypass passage 322 and releases the absorbed heat from the condenser 115. Thus, the working fluid cooled by the condenser 115 is supplied to the gear pump 111 for cooling the gear pump 111.

According to the waste heat recovery system 300 of the third embodiment, when the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1, circulation of the working fluid through the first bypass passage 317 is allowed. In this case, the expander 114, the first bypass passage 317, the cooling water boiler 112 and the exhaust gas boiler 113 cooperate to form a circulation circuit. Thus, the working fluid bypassing the condenser 115 and the gear pump 111 is supplied to the cooling water boiler 112 by the expander 114 without flowing through the condenser 115 and the gear pump 111. Therefore, when the temperature of the cooling water of the engine 140 is still low as in the case where the engine 140 is started, the temperature of the cooling water of the engine 140 is raised rapidly thereby to improve the fuel efficiency of the engine 140.

The gear pump 111, which is connected directly to the motor generator 116 without any clutch therebetween, is driven directly by the motor generator 116. The upstream end and the downstream end of the gear pump 111 are made in communication with each other by the third bypass passage 322, so that the gear pump 111 circulates the working fluid through the second closed circuit B. Thus, dynamic loading of the motor generator 116 is reduced.

The working fluid cooled by the condenser 115 is supplied to the gear pump 111, so that the gear pump 111 is cooled.

If the motor generator 116 and the gear pump 111 are formed integrally with each other, the motor generator 116 is also cooled.

Since there is a hysteresis of the temperature of the cooling water that is used as the criterion for controlling the opening and closing of the first bypass passage 317 and the third bypass passage 322, the first bypass passage 317 and the third bypass passage 322 are not frequently opened and closed. Therefore, noise generation and machine deterioration due to the frequent operation are prevented.

FIG. 4 shows a waste heat recovery system 400 according to the fourth embodiment of the present invention. The waste heat recovery system 400 of the fourth embodiment differs from the waste heat recovery system 300 of the third embodiment in that an electromagnetic valve 425 is provided at an intermediate position of the third bypass passage 322 for opening and closing the third bypass passage 322 instead of the second three-way valve 323 at the other end of the third bypass passage 322. The waste heat recovery system 400 includes a Rankine cycle 410. The electromagnetic valve 425 serves as the third valve of the present invention.

When the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1, the control unit 450 operates the first three-way valve 318 so as to allow the working fluid flowed out of the expander 114 to flow into the first bypass passage 317. Then, the expander 114, the first bypass passage 317, the cooling water boiler 112 and the exhaust gas boiler 113 cooperate to form the first closed circuit A. In addition, when the electromagnetic valve 425 is opened, circulation of the working fluid through the third bypass passage 322 is allowed. Then, the gear pump 111, the third bypass passage 322 and the condenser 115 cooperate to form the second closed circuit B.

In this case, each of the first closed circuit A and the second closed circuit B does not form a separate closed circuit, but the closed circuits A and B are in communication with each other so that the working fluid is allowed to circulate therebetween. When the pressure of the working fluid is increased excessively with an increasing temperature of the working fluid flowing through the first closed circuit A including the expander 114, the surplus working fluid is transferred to the second closed circuit B including the condenser 115. Therefore, the pressure of the working fluid flowing through the first closed circuit A is prevented from being increased excessively. Any imbalance of the amount of working fluid flowing through the first closed circuit A and the amount of working fluid flowing through the second closed circuit B which may result immediately after the first bypass passage 317 and the third bypass passage 322 are opened may be resolved with a passage of time because the working fluids are allowed to circulate between the two closed circuits A and B.

The present invention has been described in the context of the above first through fourth embodiments, but it is not limited to those embodiments. It is obvious to those skilled in the art that the invention may be practiced in various manners as exemplified below.

In the first through fourth embodiments, a valve may be provided at the upstream side of the condenser 115 (or at a position that is adjacent to the inlet of the condenser 115) for cutting off the circulation of working fluid through the condenser 115. The valve serves as the fourth valve of the present invention. FIG. 5 shows a waste heat recovery system 500 according to the modification of the first embodiment of the present invention. The waste heat recovery system 500 includes a Rankine cycle 510. A valve 526 that serves as the fourth valve of the present invention is provided at a position that is adjacent to the inlet of the condenser 115 for cutting off the circulation of the working fluid through the condenser 115. When the temperature of the cooling water of the engine 140 obtained by the temperature sensor 160 is lower than the first predetermined value Th1, the control unit 550 operates the electromagnetic valve 118 so as to open the first bypass passage 117 and also close the valve 526 thereby to cut off the circulation of working fluid through the condenser 115. Thus, all the working fluid which has absorbed heat while flowing through the exhaust gas boiler 113 bypasses the condenser 115. The electromagnetic valve 118 and the valve 526 may be combined to form a three-way valve.

In the first through fourth embodiments, the engine 140 may be connected to the drive shaft 111A of the gear pump 111 via a transmission belt for driving the gear pump 111.

In the first through fourth embodiments, it may be so arranged that the working fluid flowing through the Rankine cycle 110 directly pass through the engine 140 instead of the cooling water circuit a of the engine 140. FIG. 6 shows such a waste heat recovery system 600 according to another modification of the first embodiment of the present invention. The waste heat recovery system 600 includes a Rankine cycle 610. A first heat exchanger 212 is formed integrally with the engine 140 for exchanging heat between the working fluid and the engine 140. A temperature sensor 260 is provided for measuring the temperature of the engine 140 instead of the temperature sensor 160 of the first embodiment. The control unit 150 controls the operation of the electromagnetic valve 118 in accordance with temperature information obtained by the temperature sensor 260, thereby to open or close the first bypass passage 117. The other end of the second bypass passage 119 is located at the downstream side of the gear pump 111 and the upstream side of the first heat exchanger 212. The rest of the structure of the present modification is substantially the same as that of the first embodiment.

When the temperature of the engine 140 obtained by the temperature sensor 260 is a first predetermined value Th1′ or higher, the control unit 150 causes the electromagnetic valve 118 to close thereby to close the first bypass passage 117. When the temperature of the engine 140 obtained by the temperature sensor 260 is lower than the first predetermined value Th1′, on the other hand, the control unit 150 causes the electromagnetic valve 118 to open thereby to open the first bypass passage 117, thus allowing the working fluid to circulate through the first bypass passage 117. The expander 114, the first bypass passage 117, the gear pump 111, the first heat exchanger 212 and the exhaust gas boiler 113 cooperate to form a circulation circuit. Thus, the working fluid bypassing the condenser 115 is delivered by the expander 114 and then supplied to the first heat exchanger 212. Therefore, when the temperature of the engine 140 is still low as in the case of a start-up of the engine 140, the temperature of the engine 140 is raised rapidly thereby to improve the fuel efficiency of the engine 140.

In the first through fourth embodiments, the exhaust gas boiler 113 may be provided for exchanging heat between the working fluid and a second intermediate medium exchanging heat with exhaust gas. FIG. 7 shows a waste heat recovery system 700 according to yet another modification of the first embodiment of the present invention. The waste heat recovery system 700 includes a Rankine cycle 710. An exhaust gas boiler 214 is provided for exchanging heat between the exhaust gas and a second intermediate medium of a second intermediate medium circuit β. A second heat exchanger 213 is provided for exchanging heat between the working fluid and the second intermediate medium. The second intermediate medium flows in the second intermediate medium circuit β in which the exhaust gas boiler 214, a pump 170 and the second heat exchanger 213 are provided.

In the first embodiment, the expander 114 may be of a variable capacity type. When the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1, the gear pump 111 and the expander 114 are operable so as to transfer the same amount of working fluid, respectively, by adjusting the capacity of the expander 114. The waste heat recovery system 100 having such a variable capacity type expander 114 may dispense with the second bypass passage 119.

In the third embodiment, a clutch may be provided between the gear pump 111 and the motor generator 116. When the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1, the clutch may block the transmission of the rotary drive force from the motor generator 116 to the gear pump 111, thereby preventing the gear pump 111 from being driven. The waste heat recovery system 300 having such a clutch may dispense with the third bypass passage 322 and the second three-way valve 323.

Referring to FIG. 8 showing the waste heat recovery system 800 according to the modification of the third embodiment of the present invention, the one end of the third bypass passage 322 is located at the upstream side of the gear pump 111 and the downstream side of the condenser 115. The waste heat recovery system 800 includes a Rankine cycle 810.

In the third embodiment, the waste heat recovery system 300 may dispense with the third bypass passage 322 and the second three-way valve 323. When the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1, the gear pump 111 which is then operated with the upstream side thereof closed is idled thereby to cause power loss. However, the structure of the waste heat recovery system 300 is simplified.

In the third embodiment, an additional passage may be provided for communication between the first bypass passage 317 and the third bypass passage 322. Such additional passage allows the communication between the first closed circuit A which includes the expander 114, the first bypass passage 317, the cooling water boiler 112 and the exhaust gas boiler 113, and the second closed circuit B which includes the gear pump 111, the third bypass passage 322 and the condenser 115. Therefore, this modification offers the same effects as the fourth embodiment.

In the third embodiment, when the temperature of the cooling water of the engine 140 is lower than the first predetermined value Th1 and when the first bypass passage 317 and the third bypass passage 322 are both opened, at least one of the bypass passages 317 and 322 may be temporarily closed. In this case, the first closed circuit A which includes the expander 114, the first bypass passage 317, the cooling water boiler 112 and the exhaust gas boiler 113, and the second closed circuit B which includes the gear pump 111, the third bypass passage 322 and the condenser 115 are in communication with each other. Therefore, this modification also offers the same effects as the fourth embodiment.

In the first through fourth embodiments, controlling to open and close the bypass passages may be performed in accordance with any parameters that represent the temperature of the cooling water of the engine 140 instead of the temperature obtained by the temperature sensor 160. 

1. A waste heat recovery system of an engine comprising: a Rankine cycle allowing a working fluid to circulate therethrough, the Rankine cycle having: a first heat exchanger for exchanging heat between the working fluid and the engine or a first intermediate medium exchanging heat with the engine; a second heat exchanger for exchanging heat between the working fluid and an exhaust gas of the engine or a second intermediate medium exchanging heat with the exhaust gas of the engine; an expander for expanding the working fluid to recover mechanical energy; a condenser for condensing the working fluid; a first bypass passage connected to the Rankine cycle for allowing the working fluid to pass therethrough, wherein one end of the first bypass passage is located at an upstream side of the condenser and the other end of the first bypass passage is located at a downstream side of the condenser with respect to flow direction of the working fluid; a first valve for opening and closing the first bypass passage; and a control unit for controlling operation of the waste heat recovery system, wherein when temperature of the engine or the first intermediate medium is lower than a first predetermined value, the control unit opens the first valve.
 2. The waste heat recovery system according to claim 1, wherein the one end of the first bypass passage is located at a downstream side of the expander and the other end of the first bypass passage is located at an upstream side of the first heat exchanger.
 3. The waste heat recovery system according to claim 2, further comprising: a pump provided for pumping the working fluid, wherein the other end of the first bypass passage is located at an upstream side of the pump; a second bypass passage connected to the Rankine cycle for allowing the working fluid to pass through the second bypass passage, wherein one end of the second bypass passage is located at the upstream side of the pump and the downstream side of the expander and the other end of the second bypass passage is located at a downstream side of the pump and the upstream side of the first heat exchanger; and a second valve for opening and closing the second bypass passage, wherein the second valve opens when pressure of the working fluid at the upstream side of the pump is higher than pressure of the working fluid at the downstream side of the pump.
 4. The waste heat recovery system according to claim 3, wherein the second valve is a check valve.
 5. The waste heat recovery system according to claim 2, further comprising a pump for pumping the working fluid, wherein the other end of the first bypass passage is located at a downstream side of the pump.
 6. The waste heat recovery system according to claim 5, further comprising: a third bypass passage connected to the Rankine cycle for allowing the working fluid to pass through the third bypass passage, wherein one end of the third bypass passage is located at the upstream side of the pump and the downstream side of the one end of the first bypass passage and the other end of the third bypass passage is located at the downstream side of the pump and the upstream side of the other end of the first bypass passage; and a third valve for opening and closing the third bypass passage, wherein when the temperature of the engine or the first intermediate medium is lower than the first predetermined value, the control unit opens the third valve, wherein the expander, the first bypass passage, the first heat exchanger and the second heat exchanger cooperate to form a first closed circuit, and wherein the pump and the third bypass passage cooperate to form a second closed circuit.
 7. The waste heat recovery system according to claim 6, wherein the one end of the third bypass passage is located at the upstream side of the condenser.
 8. The waste heat recovery system according to claim 6, wherein the first closed circuit and the second closed circuit are in communication with each other so that the working fluid is allowed to circulate between the first closed circuit and the second closed circuit.
 9. The waste heat recovery system according to claim 1, further comprising: a pump for pumping the working fluid; a motor generator for driving the pump and converting the mechanical energy generated by the expander into electric power; and a clutch provided between the expander and the motor generator, wherein the one end of the first bypass passage is located at a downstream side of the second heat exchanger and an upstream side of the expander and the other end of the first bypass passage is located at an upstream side of the pump.
 10. The waste heat recovery system according to claim 1, wherein when the temperature of the engine or the first intermediate medium is a second predetermined value that is higher than the first predetermined value or higher, the control unit opens the first valve.
 11. The waste heat recovery system according to claim 6, wherein when the temperature of the engine or the first intermediate medium is a second predetermined value that is higher than the first predetermined value or higher, the control unit closes the first valve and the third valve.
 12. The waste heat recovery system according to claim 1, further comprising a fourth valve connected to the Rankine cycle at the upstream side of the condenser and a downstream side of the expander for cutting off the circulation of the working fluid through the condenser, when the temperature of the engine or the first intermediate medium is lower than the first predetermined value, the control unit closes the fourth valve. 